Our brave police risk their lives to keep us safe, but they can't prevent every attack

Metropolitan Police Service Acting Commissioner Craig Mackey salutes during a one minutes silence for their fallen officer, Pc Keith Palmer, outside New Scotland Yard in London - EPA/ANDY RAIN

The attack in London on Wednesday shocked even the most experienced counter-terrorism officers. You always know the stakes are high, you know that many innocent people will be killed if an attack gets through and you work tirelessly to prevent it.

Counter-terrorism officers can have as many as 40 investigations running at any one time, and this means they have to prioritise who to watch. It is not an exact science assessing which extremists pose the greatest threat.

Imagine for a moment what police officers would have been thinking as they put their uniforms on Thursday, knowing that a friend and colleague had been murdered just the day before. It is a time to stop and reflect upon the bravery of our police and to support the men and women who put themselves in harms way to keep us all safe.

The vast majority in our country, such as Pc Palmer, are unarmed. When confronted by a man with a knife, they are trained to run towards the threat, not away. They have to put their lives at risk so others in Parliament can go about theirs freely.

The focus of the investigation by the Counter Terrorism Command (known as SO15) will continue to be about ensuring that other terrorists are not planning to carry out follow-up attacks. More arrests and searches may follow those carried out yesterday.

Sources and informants in the Muslim community will be spoken to. Very few terrorists operate entirely alone; it will be surprising if there were not others who were at least aware of an intention to carry out an attack.

All terrorists live in communities and someone will probably have known something about this attack. Digital forensic experts will scrutinise phones and other devices, searching for clues of associates and contact with known terrorists overseas. CCTV footage from across London will be seized and scrutinised to track the journeys of the suspect’s vehicle in the past few weeks and months. The intelligence agencies will trawl through their databases looking for links to this now-reactive police investigation.

Critics will conveniently forget that at least 13 terrorist attacks have been stopped in the past few years with scores of suspects convicted of serious terrorist offences. They will not consider in context the 18 attacks that have taken place in France in the past past two years, five in Germany and five in Belgium with the loss of hundreds of lives. We should acknowledge that despite everything that happened in Westminster on Wednesday, London remains one of the safest capital cities in Europe.

Richard Walton was the Met’s Counter Terrorism Command (SO15) head and is now a Senior Associate Fellow at the Royal United Services Institute